Eukaryotic transcription is regulated by multiprotein-complexes, known as coactivators, which alter chromatin or recruit general transcription factors to facilitate gene expression. Most promoters studied use several coactivators for normal transcriptional regulation. Interestingly, some results suggest that coactivator complexes are functionally redundant in vivo, even though the complexes themselves have distinct biochemical activities. SAGA and Swi/Snf, two coactivators conserved throughout eukaryotes, have been shown to be redundant at particular genes in S. cerevisiae. SAGA has multiple activities including histone acetylation, deubiquitination, and recruitment of general transcription factors while Swi/Snf is important for nucleosome remodeling. Individually, these complexes are not essential for viability in S. cerevisiae, yet double mutants that have lost both complexes are inviable, suggesting functional redundancy. The cause of this redundancy is poorly understood. In this proposal, the role of redundancy in transcriptional regulation will be addressed through two complementary approaches, using S. cerevisiae as a model organism. First, the consequences of loss of both coactivators will be assayed, using a rapid, inducible protein-depletion system known as AID. Use of this conditional protein depletion system will allow comparisons of changes in genome- wide transcriptional profiles and coactivator localization following depletion of complexes, either singly or together. This approach will determine which of the changes in transcriptional output underlie the double mutant lethality. Second, a screen for mutants that suppress the lethality of SAGA and Swi/Snf double mutants will be conducted. The identification and analysis of suppressor mutations will provide significant insight into the mechanism allowing these two complexes to act redundantly. Given the conserved nature of these coactivators and their roles development and diseases, including cancer, understanding the mechanism behind their redundancy remains an important but yet unexplored area of eukaryotic gene expression.